1. Field of This Invention
This invention relates to a process for the production of benzotrifluoride.
2. Prior Art
Whitmore, Frank C., "Organic Chemistry," 2nd Ed., D. Van Nostrand Co., Inc., (1951), at page 622, teaches that benzotrichloride, with suitable reagents such as AgF or HF, gives benzotrifluoride. At page 275, Whitmore, ibid., teaches that benzotrifluoride results when benzotrichloride is treated with Ag or Sb fluoride.
Compounds which carry one or more trichloromethyl groups on an aromatic nucleus can be converted using hydrogen fluoride into the corresponding trifluorometyl compounds -- see Houben-Weyl, "Methods of Organic Chemistry," vol. 5/3, pp. 121 to 123. However, the industrial practice of such reaction results in large amount of formation, with the result that only irregular and small yields are achieved.
German published Application No. 1,618,390 describes a continuous process which occurs in a flow pipe that involves the use of an excess of hydrogen fluoride of at least one third of the theoretical (stoichiometric) quantity. However, the use of a reaction pipe for the subject reaction has several disadvantages since the developing hydrochlorid acid gas forms a gas cushion in the reaction space thereby considerably reducing the effective reaction volume. Furthermore, in order to achieve the necessary intensive intermixing, a flow of great turbulence must be striven for in the reactor. This has the disadvantage that the narrow cross sections of the pipe, which are necessary for the purpose of this measure, are inclined to plug up as a result of the formation of resin.
German published Application No. 1,965,782 proposes, in order to avoid the undesirable formation of resin, adding hexamethylenetetramine to the reaction mixture. Preferably 5 mole of hydrogen fluoride are used per mole of starting compound and trichloromethyl group. The reaction time according to the example is about 48 hours.
In order to prevent corrosion, a proposal has been made in German published Application No. 2,161,995 to carry out the reaction at a pressure of about 3 to 20 atm. and at a temperature of about 20.degree. to 60.degree.C. For the successful execution of the reaction however, one must use an excess of hydrogen fluoride in a quantity of at least 3 times the quantity required theoretically (stoichiometrically).
The mentioned processes have the disadvantages of needing a large excess of hydrogen fluoride. Both for economic reasons and for reasons of environmental protection, these excesses must be recaptured as much as possible in order to be able to feed them again to the reaction. When slightly smaller excesses of hydrogen fluoride are used one finds that the reaction times are in the range of several days, which of course is likewise detrimental to the economics of the process.